How To Build a Stir Plate

by Rob | March 13th, 2009

Stir plate in action

This how-to describes all the steps I took to build my stir plate. This idea isn’t my own; my buddy recently built one a few months ago and there are some other Do-It-Yourself articles scattered around the Internet. Using a stir plate is one of the best methods for increasing the yeast count in your starter. There is a great article on Yeast Propagation and Maintenance by MB Raines that discusses the advantages of culturing yeast using a stir plate.

The cost to build it was near $40 dollars. The most expensive component was the enclosure; you could use Tupperware or craft one out of wood if you wanted to knock off additional cost. Also, I wanted to use an LED, and an On-Off switch; they are cool but not required. If you decide not to use those components you could do away with the circuit board as well. This probably goes without mentioning, but you’ll need some basic tools like a drill and a soldering iron to put everything together.

Parts List:

$15.50 - Enclosure; Serpac A42-Series
$03.50 - Copper Clad Breadboard; Surplus store
$06.00 - Potentiometer; NTE 500 ohm single-turn / linear
$07.00 - DC Fan; Link Depot 2,500 RPM 12VDC 2.4W
$03.00 - AC to DC Power Adapter; SS-101, 300mA 12V
$01.50 - DC Power Jack; 2.1mm Philmore
$01.75 - Toggle Switch; GC Electronics On - On miniature bat handle toggle
$00.16 - Resistor; NTE 1/2W, 560 Ohm
$00.26 - LED; Linrose 20mA, 3V
$00.13 - LED Mounting Clip; Linrose
$00.00 - Rare Earth Magnet; (taken from an old hard drive)

Total: $38.80

I used Dia to draw out the circuit diagram (anyone know of good, free, circuit design software?). Dia has a mixed bag of pre-drawn circuit components, so the electronic buffs out there will probably notice I am using both US and European style components. In any case, here is my simple stir plate circuit:

Magnet harvest

Constructing this was really pretty simple so I’ll keep this brief.

If you’re going to harvest a rare earth magnet from your hard drive you’ll almost certainly need a special set of screw driver bits to open it up; or you know, a hammer and something to act as a wedge — that will do the trick too. The magnets are located at the base of the arm.

Size up the components and determine placement. Make all the necessary holes and modifications to mount everything. I drilled holes in the panels of the enclosure for the power switch, potentiometer, and the LED. The enclosure’s rivet holes made the fan sit awkwardly, so drilled some holes in the circuit board, and then mounted the fan to circuit board, which was screwed into the rivet holes.

Modified enclosure

Remove any labeling on the center of the fan, and glue the magnet in the center of the fan; I used Gorilla Glue. When I first did this I glued 2 magnets side by side. You don’t want to do this (at least not how I had them aligned). Using 2 magnets side-by-side made the stirbar dance around wildly. It wasn’t effective at all.

The fan you buy will most likely have 2, 3, or 4 wires. You’ll only need two (power and ground) for this setup so the least number of wires the better. I went with a very simple approach — I am using potentiometer to behave as a rheostat to vary the resistance. The potentiometer will increase or decrease the speed of the fan depending on the direction turned. As a side note, it occurred to me that Pulse Width Modulation (PWM) using 555 timer is probably a better approach if I ever wanted to build some kind of super stir plate used during primary fermentation, but for yeast starters it doesn’t really matter.

One magnet worked better than two

If you went to the trouble of using a circuit board, you’ll probably want to solder pin headers to circuit board that attach to the LED, switch, power, and potentiometer using wires. On the reverse side of the board you can then use a wire wrap tool, or soldering iron to wire the circuit together.

That’s pretty much it! Here is the stir plate in action:

Below is a number of photos a took when I made it.